Acoustic mine mechanism

ABSTRACT

1. A system for providing an improved triggering function for firing of a ne from the acoustic signature of a ship passing thereover or in the vicinity thereto comprising means for detecting the envelope of a portion of the audio signature spectrum of the ship, means for deriving a signal corresponding to the logarithm of said detected envelope signal, means responsive to said logarithm signal for providing a signal represntative of the first time derivative thereof, means for subsequently obtaining a signal simulating the second time derivative of said logarithmic signal, means for inverting the phase of said second time derivative signal, means for providing a multiplication of said first and second time derivative signals, and means responsive to said multiplied signals for actuation of a mine detonating circuit.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to an acoustic discriminator system for aircraftlaid ground mines and an improved method of triggering a mine firingcircuit. More particularly, the invention is concerned with providingsharper athwartship amplitude fall-off curves or characteristics formine firing circuit actuation in which the firing peak is advanced withrespect to the peak of the second derivative of the logarithm of theinput signature. This invention additionally provides depth compensationfor different depths of submergence of the mine while substantiallypreventing early and late misfirings of the mine mechanism such as areinherent in prior systems utilizing sensitivity to the audio amplitudefall-off characteristics of the signals received abeam from the ship andwhich signal curves are normally too flat to provide a reliable firingor triggering function for mine actuation.

It is a purpose of the invention to eliminate the loudness of the ship'sacoustic signal as a factor governing the triggering of the minemechanism.

More specifically the invention utilizes a particular predeterminedfrequency band of the sound spectrum of a ship's audio signature andutilizes the product of the first and of the second derivative of thelogarithm of the acoustic pressure signal with respect to time as atriggering function for mine firing.

Certain disadvantages are inherent in the prior mine triggering methods,which disadvantages include a flatter athwartship amplitude fall-offcurve than is desirable for reliable operation. This unfavorablesituation introduces a great number of early and late misfirings of themine. The prior mechanisms based on the utilization of the rectifiedaveraged amplitudes of the pressure signal and the first derivative withrespect to time or the first and second time derivatives of therectified and averged amplitude of the received pressure signal havepatterns which are not independent of the loudness of the ship. It is afeature of the instant invention to provide a triggering function formine firing which is essentially independent of the ship's loudnessconstant and which results in a greater adaptability and better controlof the firing pattern.

While the instant invention is hereinafter described with respect to anembodiment which utilizes electronic circuitry for obtaining atriggering function based on the foregoing relationships it is to beunderstood that it is within the province of one skilled in the art toutilize mechanical or hydraulic systems or combinations of circuitry toprovide one or all of the intermediate functional relationships forobtaining the desired triggering function and without departing from thescope of the instant invention.

In a generalized form of the instant invention the sound signals pickedup by the acoustic transducers are passed through suitable amplifyingstages as required to provide necessary gain for the succeedingplurality of operations to which the detected signal is subjected inorder to provide the desired triggering function. These functionsgenerally include apparatus or circuits for providing; a band passfilter arrangement for attenuating certain low frequency and highfrequency components of the input signal, a second detector stage forsubjecting the signal to rectification to detect the envelope of theship's audible signature, passing the signal through a stage having alogarithmic transfer characteristic for obtaining the logarithm of thesignal envelope, thereafter subjecting the output of this stage whichprovides the logarithm of the signal to a smoothing filter prior to theamplifying of this output, as required, to provide a signal of suitablelevel for multiplication with a phase inverted signal representing thesecond derivative of the signal with respect to time. The sequencefurther includes subjecting a portion of the signal output at the lastmentioned amplification stage to a differentiating network for thetaking of the second derivative of the logarithm of the signal withrespect to time, thereafter inverting and amplifying this portion of thesignal in an additional amplifying stage in a manner for multiplying inan output stage of the system.

This output stage may incorporate a sensitive plate current type relayfor application of the triggering function output to the mine mixer forutilization.

It is a feature of this invention to provide an improved triggeringfunction characteristic for actuation of a mine firing circuit from theaudible signature of a ship passing thereover which function provides atriggering signal which is independent of the amplitude of the shipemitted sound.

One object of this invention resides in providing an improved method oftriggering a mine firing circuit by utilizing the product of the firstand second derivatives of the logarithm with respect to time of theship's acoustic signature.

Another object of this invention resides in the provision of a new andnovel combination of electronic circuitry for successively detecting aship's underwater audible signature, obtaining the envelope of theship's acoustic signature, obtaining a signal equivalent to the takingof the logarithm thereof, obtaining a signal representative of the firstdifferential of said logarithmic signal with respect to time, thereafterobtaining a signal characteristic of the second derivative of saidlogarithmic signal with respect to time and obtaining an output signalsimulating the product of said first derivative signal and said secondderivative signal.

Another object resides in the provision of an improved system for firinga mine from a ship's audible signature which substantially overcomes allthe foregoing difficulties of systems heretofore or now in general use.

Another object of the invention resides in the provision of a system fordetecting the audible signature of a ship passing over a mine whichprovides a triggering function having the advantage over prior systemsof reliably advancing the firing of the mine to a substantially shortertime than systems heretofore or now in use of a character which requirethe passage of a substantial portion of the ship over the detectingdevice.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an electronic circuit of a characterfor carrying out the method of the invention and is directed to apreferred embodiment of the instant invention;

FIG. 2 is a curve showing in representative form the amplitudecharacteristics of the underwater acoustic signatures of twocharacteristic types of ships of different degrees of loudness as theypass over a mine at the same speeds;

FIG. 3 is a curve showing the limitation of amplitude differencesbetween large and small ships or ships of different degrees of loudnessmoving at the same speeds when the logarithm of the respective signalsare compared;

FIG. 4 is a curve showing the characteristics of the first differentialof the logarithm signals with respect to time of FIG. 2 and showing howthe system becomes insensitive to amplitude variations of the audiosignal;

FIG. 5 is a showing of the characteristic curve of the second derivativewith respect to time of the logarithmic signal of FIG. 3;

FIG. 6 is a group of curves showing the curve of FIG. 4, the invertedcurve of FIG. 5, and a curve representing the product of these twosignals of FIGS. 4 and 5, thereby illustrating how the firing time ofthe mine is advanced by the multiplication of the first and seconddifferential signals; and

FIG. 7 is a diagrammatic representation of the geometric relationshipsof interest between a mine and a ship passing in the vicinity thereto.

Referring now to FIG. 1 there is shown a simplified circuit diagram ofan acoustic discriminator comprising one embodiment of the instantinvention. The circuit consists essentially of the followingsuccessively arranged elemental circuits shown in the dotted outlineblocks in which a signal transducer such as shown at 11 as a crystalmicrophone is connected to the input of an electronic amplifier 13 afterpassing through a band-pass filter network indicated by the block 12.The band-pass filter comprises resistive and capacitative elementsconnected in a configuration providing both a low frequency attenuationnetwork and a high frequency attenuation network with the componentsthereof selected in a well-known manner to pass a desired portion of thefrequency spectrum of the underwater audio signals produced by a shippassing over a mine or in the immediate vicinity thereto.

The amplifier section 13 is shown to comprise two conventional triodetube resistance-capacitive coupled stages V₁ and V₂ connected incascade. The output at the plate of the second stage V₂ is rectified bythe voltage doubler circuit 14 which, advantageously, may employ barriertype rectifiers. If desired, a voltage sensitive relay 15 may beincorporated in this stage for operation by a high rate of change ofthis voltage for providing anticountermine protection and temporarydisablement of the mine firing circuits. Signals having high rates ofchange in voltage effect closure of relay contacts which disables theoutput circuit of the mine not shown.

The voltage appearing at the output of the voltage divider circuit isthen applied to a network utilizing a barrier type rectifier 16 having aresistance variation such that the voltage developed across thisrectifier or semiconductor is a logarithmic function of the currenttherethrough. Block 17 shows this logarithmic rectifier circuit incombination with a smoothing filter which is utilized to reduceundersirable effects of the erratic variations in the ship's acousticoutput.

The subsequent stage of this system at block 18 comprises means forobtaining the first derivative of the logarithmic signal characteristicsby a simple resistance-capacitance network as shown. The values of thetime constant are so chosen that a sufficiently good derivative isobtained without introducing excessive attenuation of the signal.

Since these two requirements conflict, the final design values chosennecessarily represent a compromise, however it has been determined fromsimulation studies that decreasing the time constant to extremely lowvalue does not significantly improve the performance characteristics.

The signals from this differential network 18 appearing across thestepped attenuator 19 are amplified by the subsequent triode amplifyingstage V₃ shown in block 21. The gain of this stage is determined by theposition of the moving contact 20 of the stepped attenuator 19 and for apurpose as will hereinafter become apparent. The amplifier output oftube stage 21 is utilized by being applied through two separate paths,to a subsequent stage V₅, the signal of one path isresistance-capacitance coupled to the first grid of the pentagridmultiplier tube V₅ of block 25. The signal from V₃ for the second outputpath is passed through a second resistance capacitance differentiatingnetwork 22 to the grid of the amplifying and inverting stage V₄ in block23. The output of amplifying stage 23 is applied to the second signalgrid of the pentagrid multiplier stage 25 to additionally control theconduction of the tube in a manner whereby the current flow throughplate circuit relay 26 corresponds to the desired triggering function.This current flow represents the product of the first time differentialof the logarithm of the acoustic signal envelope and the second timederivative of the logarithm of this same signal envelope.

It is deemed apparent from the foregoing that amplified voltagescorresponding to the first and second time derivatives of the logarithmof the ship's signature are available at the plates of the tubes V₃ andV₄ respectively with the output of V₄ inverted in phase with respect tothe signal from tube V₃. It will now be apparent from a consideration ofthe curve of FIG. 4 that the second time derivative signal must bepositive for usage of this signal. This is accomplished by passing thesignal through the additional amplification stage 23 of tube V₄. Inorder to utilize these outputs as a product it is necessary to multiplythese two voltages in a simple manner. This approximate multiplicationis accomplished as hereinbefore stated by applying the signalrespectively to the first and second signal grids of the pentagridconverter type tube V₅ in which the flow of plate current issubstantially proportional to the product of the two voltages within thelimited range required and for positive voltages. The voltages areintroduced to the grids by resistive-capacitive coupling networks havinglarge time constants as compared to the time constants used in thedifferentiating networks. The relay in the plate circuit of tube V₅provides the contact closure required of the discriminator to allow anactuation of the mine circuit.

The theoretical localization is considerably reduced as the depth ofsubmergence of the mine increases since the length of the radius factorR FIG. 7, from the mine to the ship changes more slowly as distanceabeam increases at greater depths. Some form of depth compensation isthus desirable. This relationship will be apparent from FIG. 7 and themathematical presentation hereinafter set forth. The depth compensationfunction, however, is obtained by the stepped attenuator of FIG. 1 at19, 20, which changes the effective gain in the first derivative stagenetwork 18 in definite steps. Although the structure is not shown incomplete form, the attenuator may be operated by bellows structuregenerally indicated at 27 by the hydrostatic pressure of the water inwhich the mine is planted. Also it is desirable to provide somecompensation for the speed of the vessel. This is obtained by means ofthe continuously variable attenuator 24 in one of the grid circuits ofthe multiplication tube. This attenuator may be ganged to the rotorshaft for the multiple cam switches of the mixer mechanism or drivenfrom the constant speed D.C. motor utilized in the mixer drive of themine mixer mechanism. It is connected in an arrangement providingmovement of the tap of the variable resistor. This movement may be arotary motion occurring at the time the first detectable influence towhich the mine is sensitive, such for example as a magnetic "look " ispresented to the mine mixer. The mixer may comprise a constant speedtiming arrangement for providing electrical switching in variouscircuits of the mine in a predetermined time sequence. The magnetic"look" occurs as the bow of the ship enters the field of magneticsensitivity of the mine. The greatest attenuation of the detectedacoustic signal occurs at the start of rotation of the aforementionedvariable resistance and is decreased according to an approximate cubiclaw so that the maximum sensitivity is provided for the slowest vessels.The resistance of the potentiometer requires a cubic relationship withtime since speed occurs in the fall-off equation to the third power. Inthis manner it is possible to provide for speed variations of ships. Inpractice, however, partial compensation rather than completecompensation is used since the length of ships to which the mechanism issubjected is not a constant. Where the instant triggering functioncircuit is utilized for a combination influence mechanism "looks" suchas the aforementioned magnetic field influence are received from otherinfluence mechanisms during the passage thereover of the bow of theship. A variable attenuator for speed is therefore utilized. For othertypes of mines of a character not having magnetic or pressurediscriminators, the value of the speed compensation potentiometer willbe set to a fixed value as determined by the expected ship traffic.

The operation of the circuit will become more apparent when taken inconsideration with the mathematical presentation hereinafter set forthwith the mathematical representation of a ship's sound signature. It iswell known that the audio spectrum of a normal ship's sound is complexsince energy components are distributed over a wide range of frequenciesand amplitudes. It is therefore desirable to initially select afrequency band or bands which will yield optimum localization for thetype of vessel under consideration. The localization as herein used isdefined as a degree of discrimination obtained by a mechanism as afunction of the distance abeam and, as plotted, is shown commonly as anathwartship amplitude fall-off curve.

Referring now to FIG. 7 there is a showing of the geometric relationshipof interest for a ship passing a ground mine wherein the followingdesignations in the mathematical analysis relate to certain of thereference characters as follows:

V=Sound pressure

R=Radial distance from the mine to the ship

K=Ship's loudness constant with the sound energy output of a shipassumed to be constant during passage.

S=On-course distance

N=Least distance of approach of ship to the mine=√(b² +d²)

t=Time which is assumed to be 0 at the time corresponding to the leastdistance of approach

c=Speed of vessel

α=Fall-off exponent

b=Distance abeam which is the normal distance from ship's course to apoint on the surface directly over the mine.

d=Depth of mine

The envelope of the sound pressure from a given ship can be representedapproximately by the function ##EQU1##

The typical shape of the rectified signal received from a hydrophoneafter filtering with a band-pass filter to obtain optimum localizationby suitable selection of the frequency range is shown by curves A and Bof FIG. 2. The maximum amplitude of the signal, in general correspondsto the least distance of approach of the vessel to the hydrophone, ormore strictly to the least distance of approach of the source of soundin the vessel to the hydrophone. From formula (1) the constant K in thisequation is hereinafter designated as the ship's loudness constant andequation (1) may be written as:

    K=VR.sup.α                                           (2)

The loudness constant can therefore be expressed as the product of thesound pressure and the distance at which the source of noise is observedto some power α. Suitable values of the exponent, α required to obtainan approximate mathematical fit with experimental data by this equationyield values ranging from 0.8 to 2.2. The value of α varies with bottomconditions and with water depth. The choice of the frequency bandutilized for optimum localization is partially dependent on the valuesof α obtained from experimental data. It has been determined by testthat a satisfactory value of α can be approximated as unity. The ship'sloudness constant K however varies greatly from ship to ship and isproportional to the total amplitude of sound produced by the ship and,consequently, varies also with the speed of the vessel. The signaturesof a few ships depart from these simplifying assumptions due to theexistence of several sources of sound in the vessel and for otherreasons, however by proper design of the smoothing filter and choice ofoperating frequency band, some of the effects of these anomalies can beminimized in the discriminator design. The characteristic envelopeappearance of the signals from two ships having different loudnesscharacteristics and after passing the received signal from thehydrophone through a band-pass filter, rectifier and smoothing filter isas shown by curves A and B on FIG. 2.

The prior art methods of triggering used by certain of the existingmechanisms, in general utilize the magnitude of the envelope of thesound pressure variation as shown in FIG. 2 and require that a givenminimum value of sound pressure exist for a given period of time inorder to trigger the mine. However, for very slow vessels, the rate ofchange of the sound pressure variation is not appreciable.

From equation (1), it is apparent that the maximum value of soundpressure occurs at t=0 or when the ship is nearest to the mine. Thefollowing equation is then used for determining the athwartshipamplitude fall-off curve. ##EQU2##

As a consequence of the above, the mine firing is dependent on K, theship's loudness constant, and the value of the least distance ofapproach of the ship to the mine. Since K varies greatly from ship toship, it is impossible to have a highly localized firing pattern withthis type of triggering function. Since the value of α is usually aboutone, an inverse first power athwartship amplitude fall-off pattern isexpected in general. The mathematical representation for an idealsignature is developed in the foregoing. It is possible however toderive other function from this ideal signature which may be more usefulor desirable as a trigger function. The simplest of these is the rate ofchange of sound pressure with time.

Referring now to the signatures of two vessels shown in FIG. 2 havingdifferent rates of change as illustrated by the tangents drawn to thesecurves, the tangent lines represent the maximum rates of change of soundpressure for the respective signatures. These two target signaturesdiffer because of possible variations in K, α, C and R. Differentiatingequation (1) to show the rate of change, mathematically ##EQU3## whereinthe maximum value of DV/dt of interest is that which occurs when##EQU4##

If α is assumed =1, ##EQU5## The obvious conclusions to be obtained fromexpression (3) are that if a voltage proportion to (DV/dt) were used totrigger the firing of the mine, the mine firing would be dependent,theoretically, on the ship's loudness constant K, the fall-off exponentα, the speed of the vessel, and the least distance of approach. Sincethe ship's loudness constant K varies greatly from ship to ship anddepends on the amount of sound generated, the rate of change isdependent upon the magnitude of the sound output of the vessel. Thus, itis impossible, or at least entirely unsatisfactory to secure asatisfactory localized firing pattern with either the (V) or the (DV/dt)types of triggering functions. Therefore, to make any appreciableimprovement over this type of discriminator design, it is essential thata means for obtaining a triggering signal that is practicallyindependent of the ship's loudness constant K be obtained.

The subsequent discussion is concerned with the above intermediatefunction of the instant triggering function. After the foregoingrectifying and filtering of the signal other characteristics aredeveloped which are based on the rates of change with time or thelogarithm of the sound pressure.

The rectified and filtered signals of the sound pressure for two vesselshaving values of sound pressure which differ by a factor of 2 areillustrated generally by FIG. 2. The curves A' and B' shown in FIG. 3result from having obtained the logarithm of the curves A and Brespectively as shown on FIG. 2. This operation produces a set of newcurves, now having the same shape, i.e., by vertical displacement, onecould be superimposed over the other. These curves are representative ofvessels having the same course, speed, etc. and differing only by theamount of sound produced, or in mathematical terminology have adifferent value of K in equation (1).

FIGS. 4 and 5 show curves A", B" and A'", B'" respectively for the firstand second time derivatives of the amplitude characteristics shown inFIG. 3. These derivates are independent of the scale factor of theoriginal amplitude of the sound pressure and are identical as shown inrepresentative form by the solid line curves A", A'" and the dotted linecurves B" and B'". Therefore, the mine firing patterns, or localization,obtained from these derived functions must also be independent of theamplitude of the sound pressure emitted by the ship. This is equivalentto stating that the magnitudes of any number of derivatives of thelogarithm of the sound pressure are independent of the value of K informulae (1).

The value of the first derivative of the logarithm is zero at the timecorresponding to the least distance of approach of the ship to the mine.At this time the second derivative has a maximum value. Mathematicallydifferentiating equation (1) ##EQU6## Differentiating again, ##EQU7##

One of the primary objects to be attained in the design of an acousticdiscriminator is a good athwartship amplitude fall-off pattern. If thesecond derivative of the logarithm were ued as the triggering function,the athwartship amplitude fall-off would vary inversely as the secondpower of the least distance of approach as will now be shown. Fromequation (6) D² lnV/d² t has a maximum at t=0. Substituting t=0 inequation (6) one obtains for the athwartship amplitude fall-off curve##EQU8##

A comparison of the athwartship amplitude fall-off patterns representedby equations (4) and (7) shows that if the parameter K of equation (4)is replaced by the parameter C, the equations are identical except for ascale factor. The range of variation of the parameter C for mineapplications is normally from about 5 knots to about 15 knots or a ratioof about 3 to 1. Since this variation in the parameter C (3:1) isconsiderably smaller than the variation of the parameter K(approximately 20:1) which has been replaced, equation (7) for thesecond derivative of the logarithm of the sound pressure variationrepresents a considerably improved triggering function over the rate ofchange type represented by equation (4) with respect to the localizationobtainable. Similar conclusions can be drawn from a comparison ofequations (7) and (1) for the amplitude type of triggering. Thetriggering function prescribed by equation (6) reaches a maximum valueat a later time than the one for equation (3). This is an adversecondition since the sources of sound in a ship are usually aft themidship section and there is an unavoidable delay due to the smoothingfilter in a practical discriminator design. Despite the goodlocalization pattern obtained by the second derivative of the logarithm,firing would occur too late if it were used alone as the triggeringfunction.

In order to obviate the difficulties as hereinbefore outlined and tofurther improve the localization pattern, another derived function,i.e., the product of the first and second derivatives of the logarithmis utilized in the instant invention. This function provides the mostfavorable overall characteristics.

Multiplying equation (5) by equation (6) one obtains the productfunction, P, shown as curve P on FIG. 7, which is the product of thefirst and second derivatives of the logarithm of the sound pressure.##EQU9##

Differentiating and setting DP/dt=0, one obtains the quartic equation##EQU10## and solving for the time when the usable product is maximum,one obtains ##EQU11## The corresponding distance ##EQU12## is obtainedby substituting the value of t in the above equation. ##EQU13## Thus itis shown that an advance in the peak value of the product function withrespect to the peak of the acoustic signature occurs which isindependent of speed and α. This triggering function is illustrated onFIG. 6 of the drawings. Further equation show that the maximum value ofthe product varies as ##EQU14## hence a very good athwartship fall-offpattern is obtained. The firing point occurs either at the peak orearlier than the peak value depending on the magnitude of the productfunction shown.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A system for providing an improved triggeringfunction for firing of a mine from the acoustic signature of a shippassing thereover or in the vicinity thereto comprising means fordetecting the envelope of a portion of the audio signature spectrum ofthe ship, means for deriving a signal corresponding to the logarithm ofsaid detected envelope signal, means responsive to said logarithm signalfor providing a signal representative of the first time derivativethereof, means for subsequently obtaining a signal simulating the secondtime derivative of said logarithmic signal, means for inverting thephase of said second time derivative signal, means for providing amultiplication of said first and second time derivative signals, andmeans responsive to said multiplied signals for actuation of a minedetonating circuit.
 2. The system of claim 1 further characterized bythe addition of means for amplifying the signal level of said secondtime derivative signal prior to application thereof to saidmultiplication means.
 3. The system of claim 2 further including meansfor correlating the time period of operation of the system with the rateof travel of the ship providing the acoustic signature being detected.4. The system of claim 1 further including means for providingcompensation in the first derivative taking means thereof for the depthof submergence of the system in a body of water.
 5. An acoustic minefiring system comprising means for detecting a desired portion of theaudio spectrum of a ship's audible underwater signature, means fordetecting the envelope of the ship's audible signature from saidselected portion of the audio spectrum, means for providing a signalcorresponding to the logarithm of said detected envelope, means forobtaining a signal corresponding substantially to the first derivativewith respect to time of said logarithm signal, means responsive to saidfirst derivative means for obtaining a second signal correlative to thesecond derivative with respect to time of said logarithm signal, andphase inverted with respect to said first derivative signal, and meansincluding a multiplier for obtaining the product of said first and saidsecond time derivative signals.
 6. The system of claim 5 furtherincluding means for providing a phase inversion of the second timederivative signal prior to application thereof to the multiplying means.7. In a mine detection system for mine actuation in response to acharacteristic intelligence indicative of a passage of a ship over saidsystem, the combination, of an underwater audio signal detector of acharacter providing an output representing the envelope of a preselectedportion of the sound spectrum of a ship's audible signature, means forderiving a signal corresponding to the first derivative with respect totime of the logarithm of said signal, means for deriving a signalcorresponding to the second derivative with respect to time of thelogarithm of said detected envelope signal, and utilization means forproviding mine actuation correlative to the product of said first timederivative signal and second time derivative.
 8. A method of providingan improved triggering function for a mine circuit of a characterutilizing a portion of the audio spectrum of a ship passing in theimmediate vicinity thereto which comprises transducing underwater audiosignal intelligence emitted from said ship into electrical signals,filtering said transduced signals to provide a desired frequency rangefrom a portion of said audio spectrum signal, detecting the envelope ofsaid filtered signal, subjecting the detected envelope to additionalfiltering of a character for obtaining a signal similating the logarithmof said envelope, thereafter obtaining a similation of the firstderivative of said logarithmic signal, smoothing out irregularities insaid first derivative signal, thereafter obtaining a similated secondderivative signal from said first derivative signal, inverting the phaseof said second derivative signal with respect to the phase of the firstderivative signal, multiplying said first derivative signal by saidphase inverted second derivative signal, and obtaining a triggeringoutput signal for mine actuation which is proportional to the product ofsaid first time derivative signal and said phase inverted second timederivative signal.
 9. A method of providing actuation of a minedetection system by detecting a portion of the audible signature of aship passing thereover, submitting the signal to the successive steps offiltering to a selective band width, rectifying to obtain a signalcorresponding to the envelope of the acoustic signature, furtherrectifying a portion of the signal to provide a signal corresponding incharacter to the logarithmic thereof, additionally filtering thelogarithmic signal to obtain simulation of the first time derivativethereof, altering the magnitude of said derivative signal to compensatefor the depth of submergence of the mine, amplifying the thuscompensated signal to increase the level thereof sufficient to overcomethe loss introduced by subsequent steps, applying the amplified outputsignal to a signal multiplying means, deriving a signal corresponding tothe second derivative of said amplified signal, phase inverting thesecond derivative signal and multiplying by said first derivativesignal, and obtaining an output for mine firing which is proportional tothe product of said first time derivative of the logarithm of theacoustic signature and the second time derivative of the logarithmicsignal thereby advancing the firing triggering function with respect tothe amplitude peak of the acoustic signature while rendering the saidoutput independent of the audio amplitude of the ship's signature.